Immense districts and whole mountains have been built up by these minute creatures.

They have been at work for ages, and are still at work.

It is principally in the South Seas that their labors are carried on.
Near the Maldive Islands they have formed a mass whose volume is equal to the Alps.

Around New Caledonia they have built a barrier of reefs four hundred miles in length, and another along the northeast coast of Australia a thousand miles in length.

In the Pacific Ocean, islands, reefs, and islets innumerable have been constructed by them, which extend for an immense distance.
"The coral islands are called 'atolls.' They are nearly always circular, with a depression in the centre.